Tire pressure monitoring systems (TPMS) in vehicle tires typically have a separate power source, such as a battery, to which the remaining TPMS circuitry is coupled in order to power the system. Currently microelectronic structures, such as transistors and MOSFET devices, have been proposed as power switches to ensure that adequate power is provided to the system. In addition, both MEMS accelerometers and traditional mechanical structures have been used in order to detect the motion of a wheel or vehicle, which in turn provides a “wake-up” signal to power up the TPMS.
However, these systems have a disadvantage in that the microelectronic device (ASIC and MEMS) itself exhibits a stand-by current; a known TPMS ASIC has a stand-by current in the order of 500 nA at a temperature of 25° C., which increases as the temperature of the device increases. Typically, at least 50% of the power consumption over the lifetime of the TPMS battery, which is approximately ten years, is due to this stand-by current. This is the case even where the above-described motion detection arrangement is employed, as although such an arrangement helps in saving battery power, the saving is not significant in comparison to that lost.
The invention seeks to overcome the above problems associated with known devices.